1. Field of the Invention
The present invention relates generally to the field of manholes suitable for use in conjunction with the fill pipes connected to underground flammable liquid storage tanks, and more particularly, is directed to a double sealed manhole suitable to contain spillage which might occur during tank fill operations.
2. Description of the Prior Art
It is the usual practice to store flammable liquids, for example, gasoline, diesel fuel and kerosene in underground storage tanks which are generally arranged in multiple banks beneath the pavements of automotive service stations. Such underground flammable liquid storage tanks are each equipped with a vent pipe which extends from the tank to a location above grade to facilitate the flow of vapors both into and out of the tank during all periods of use, as required by good practice and by local fire codes. Each tank is also equipped with a fill pipe which extends from the top of the underground storage tank up to grade level. Most often, the upper end of each tank fill pipe terminates within an individual manhole which is set in the service station pavement and the fill pipe includes an upper nipple and suitable fill pipe cap or cover.
When making product deliveries, the manhole cover associated with each tank fill is removed in turn and the delivery truck hose is directly connected to the nipple at the top of each fill pipe for product delivery purposes. Despite the tight connection between the tank truck and the top of each fill pipe, experience has shown that this construction will not prevent spillage under all conditions of use. Spills can and have occurred due to possible leakage at the fill pipe nipple to hose coupling or perhaps by discharge of a quantity of standing liquid that may remain in the hose upon completion of the product delivery. When the hose is disconnected from the fill pipe nipple, quite often any standing liquid within the hose can escape into the manhole and from there, seep into the ground.
After extended periods of time, the ground at automotive service stations can become saturated with the product spillage to thereby create local toxicity and flammable hazards. Additionally, the spilled product could enter the local ground water aquifer and thus present a considerable health hazard.
Because of the contamination problems occasioned by spillage at or about underground flammable liquid storage tank manholes, numerous state and local governments have been increasingly enacting legislation to require positive protective construction features to control or prevent such spillage. The present invention is directed to an improved manhole construction incorporating cooperating upper and lower seals to prevent the leakage of flammable product past the manhole and into the surrounding soil.